Withhold per-subject breakdown for test split

Add arXiv paper link and citation

Restructure into dev/ (with ground truth) and test/ (images only)

Clarify that statistics describe the public subset

Restore badges as inline links

Replace badges with inline text stats

Fix badge alignment

Improve header layout

Clean up loading example

Add Dr.DocBench public subset: 986 pages, 38 BISAC subjects, 12,902 block annotations